CLASS XII ACADEMIC PROGRAM 2020 CHEMISTRY 2ND ... Online Class Study...2020/10/07 · CLASS XII...

CLASS XII ACADEMIC PROGRAM 2020

CHEMISTRY 2ND PAPER

CHAPTER 2 : ORGANIC CHEMISTRY

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LECTURE : C-07

CH� − CH� − CH� − CH��(��)

CH� − CH − CH� |

CH� ��

��

CH� − CH� − CH� − CH� − CH��

(��)

CH� − CH − CH� − CH�|

CH� ��

( ��)

CH�|

CH� − C − CH�|

CH�!"��

( ��)

The compounds with same molecular formula but different structural formula or different 3d

arrangement of atoms because of which their properties differ are called Isomers of one another

and this property is called Isomerism.

Isomerism of Organic Compounds

Chemistry 2nd Paper

Chapter 2 : Organic Chemistry

Isomerism

Structural

isomerism

Chain Isomerism

Position Isomerism

Functional Group Metamerism Tautomerism Ring Chain Isomerism

Stereo Isomerism

Geometrical

Isomerism

Optical

Isomerism

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Chain Isomerism

The isomerism that is formed due to the different structure of carbon chains is called chain

isomerism. The isomers are homologous and the carbon chain can be branched or not.

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Position isomerism

The isomerism created in the homologues due to the presence of double bonds or triple bonds in different carbons of the compound

or due to the shift in position of functional group is called poition isomerism. The position of the carbon atoms remain unchanged here.

Change in position of functional group : : : : #$%&'%

Difference in position of double bond : : : : #(%)*

Chemistry 2nd Paper


Position isomerism

Position isomers due to the substituents in Aromatic

compounds::::

Molecular formula: : : : #+%, #%$ -, (xylene)

Chemistry 2nd Paper


Functional group isomerism

Due to the presence of different functional groups in the molecules having same formula, functional group

isomerism is created. The characteristics differ a lot because of being in different homologous series

Like- ether and alcohol , aldehyde and ketone , carboxylic acid and aster

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Metamerism

The molecules of compounds in the same homologous series might have different number of carbon atoms

and the isomers created in such a way is called metamerism and the isomers are called metamers. It occurs in

ether, ketone and secondary amines

#,%)*'

#(%)*'

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Tautomerism

It is a special type of functional group isomerism which is dynamic and easily changeable. The isomers are

called tautomers.

like: (i) tautomerism of acetoacetic ester. It is also called keto enol tautomerism.

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Tautomerism

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Ring Chain Isomerism

Organic compounds having same molecular formula forming isomers of both open chain and closed chain compounds

because of the formation of their chain structure are known as ring chain isomers and the isomers created in such a way

is called ring chain isomerism.

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Determining structural isomerism

1.Determining homologues

2. after determining homologous series, establish as many carbon chains as possible by the number of

carbons given in the formula

3. in the established carbon chains, place functional group in different places

4. place hydrogen to fulfill the valency of the carbon atoms

5. finally check if a stereo isomer can be formed.

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/!0�!1�

/!0�!

/!0�! +� 3

/!0�!3

/!0�!1430

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#+%),


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#,%567


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#,%5'%


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#,%)*'


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#,%8


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#$%+'

Geometrical Isomerism or Cis-truns Isomerism

Geometrical isomerism is one type of carbon carbon double bond

stereo isomerism. substituted alkenes or the compounds with same

structural formula which have different 3d arrangement of the groups

attached with the carbons in the double bond and which have different

physical and chemical properties are called geometrical isomers and

this feature is called geometrical isomerism.

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sp2 and sp3 hybridization of carbon

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Geometrical Isomerism or Cis-trans Isomerism

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Geometrical Isomerism or Cis-trans Isomerism

HH

H

CH3 CH3

H

H

H

HCH3

CH3

H

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Optical Isomerism

Compounds having same molecular and structural formula and similar physical and chemical

properties but behaves differently with plane polarized light are called Optical Isomers and this

phenomena is called Optical Isomerism.

Plane Polarized Light

Optical Activity

Chiral Centre

Chirality

Enantiomers

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Plane Polarised Light

Nicol Prism

Polarizer

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Optical Activity

Polarimeter

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Optical Activity

To compare the optical activity of different compounds specific rotation is used. specific rotation [α] is

expressed through the equation-

�9

:=

4

Conditions of showing optical isomerismThe molecule must have a chiral carbon.

The molecule must be assymetrical with respect to chiral carbon.

The molecule and its Mirror Image will not be superimposed.

Chemistry 2nd Paper


Explanation of Optical Activity

Asymmetric Carbon AtomThe presence of assymetric carbon atom in molecule is the reason for showing optical isomerism. In a

compound if a carbon atom has four different atoms/groups attached with it, it is called an Asymmetric

Carbon and assymetric carbon is called Chiral Carbon or Chiral Centre.

Y|

X − C∗ − Z

|W

Asymmetric Carbon

CHO |

H − C∗ − OH|

CH2OH glyceraldehyde

H |

H3C − C∗ − COOH

|

OH lactic acid

H |

H3C − C∗ − COOH

|

Br 2 bromopropanoic acid

Chemistry 2nd Paper


Chiral Carbon

Chemistry 2nd Paper


Enantiomers and Racemic Mixture

Two optically active isomers with chiral carbons which are mirror image of one another but are not superimposable and

can rotate plane polarized light in the same amount to the opposite directions are called enantiomers and the phenomena

is called Enantiomerism.

The equal portion mixture of two enantiomers is called racemic mixture.

Fig: isomer of lactic acid

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DiasteromerIf two different optically active compounds with assymetric carbons are not mirror image of one another are called

diasteromer of one another.

Two diasteromers rotate plane polarized light in the same direction but in different angles. They cannot form racemic

mixture.

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Meso Compounds

In a compound even though there is presence of chiral carbon, if one portion of the compound neutralizes the

other portion’s optical activity/rotation of plane polarized light and the compound is optically inactive, it is

called meso compound.

Chemistry 2nd Paper


CLASS XII ACADEMIC PROGRAM 2020 CHEMISTRY 2ND ... Online Class Study...2020/10/07 · CLASS XII...

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